基于石英增强光声光谱的HCl痕量气体高灵敏度探测研究

Research on High Sensitive Detection of HCl Trace Gas Based on QEPAS Technology

HCl是一种有毒有害气体,对其高灵敏度探测具有非常重要的意义,然而到目前为止,采用激光光谱的手段对其探测的研究报道很少。石英增强光声光谱(QEPAS)是近年来发展起来的一种痕量气体探测技术,具有系统体积小、价格低廉、探测灵敏度高等优点。以5 000ppm HCl∶N_2混合气作为待测目标,采用输出波长为1 742.38nm的分布反馈连续波单纵模半导体激光器,开展对基于QEPAS技术的HCl高灵敏度探测研究。为了提高信噪比和简化数据处理过程,QEPAS传感器系统采用波长调制和2次谐波探测技术。研究中,首先对声波探测系统中微共振腔强声波增强特性进行了讨论,选择了"共轴"形式的声波微共振腔,并对其尺寸进行了优化,选择的微共振管长度为4mm、内径为0.5mm。实验中研究了激光波长调制深度对QEPAS系统产生的信号幅度的影响,当QEPAS系统积分时间为1s、激光波长调制深度为0.23cm^(-1)时,获得了815ppb的优异检测极限,归一化噪声等效吸收系数为7.41×10^(-9)cm^(-1)·W·Hz^(-1/2)。在后续的实验中,可在待测HCl气体中加入水汽分子,提高HCl分子的热弛豫速率,进一步提高HCl-QEPAS传感器系统的信号强度。

Hydrogen chloride （ HCl） is a toxic and harmful gas. It is of great significance to detecte it with high sensitivity. But up to now there has been a few research reports about the HCl detection using lasei spectroscopy. Quartz-enhanced photoacoustic spectroscopy （QEPAS） technology was invented not long ago. This technique uses a commercially available mm sized piezoe- lectric quartz tuning fork （QTF） as an acoustic wave transducer. A high Qfactor and a 32.7 kHz resonance frequency of the QTF improve QEPAS selectivity and immunity to environmental acoustic noise. QEPAS sensor has the advantages of high sensi- tivity, selectivity and compactness. In this paper, 5 000 ppm HC1, N2 mixture was selected as the target analyte. A continuous- wave distributed feedback （DFB） single mode diode laser emitting at 1 742.38μm was used as the exciting source and QEPAS technology was adopted as the detection method. In order to reduce the sensor background noise and simplify the data process, wavelength modulation spectroscopy and a 2nd harmonic detection technique were employed. In this research, acoustic micro-resonator （mR） in the acoustic detection module and the effect of acoustic wave improvement was discussed firstly. The ＂on beam＂ architecture of mR was selected and the length and inner diameter of the mR tubes were selected to be 4 and 0. 5 ram, respective- ly. In the experiments, the influence of modulation depth of laser wavelength on QEPAS signal amplitude was investigated. When the acquisition time was 1 sec and modulation depth was 0.23 cm-1 , a la minimum detectable concentration limit of 815 ppb was obtained. The corresponding normalized noise equivalent absorption （NNEA） coefficient was 7.41 × 10 9 cm-1 . W · Hz-1/2. In the future, the detection sensitivity can be further improved by adding water vapor into the target HCl gas to enhance the V-T （vibrational-translational） relaxation rate of HCl molecular.